A METHODOLOGY FOR CALCULATING THE MINIMUM PRESSURIZATION TEMPERATURE OF NEW BUILT HYDROPROCESSING REACTORS IN 21/4CR-1MO-1/4V LOW ALLOY STEEL

Minimum Pressurization Temperature (MPT) is the lowest temperature at which a hydroprocessing reactor can start pressurizing safely. A reliable MPT evaluation is necessary due to the susceptibility of hydroprocessing reactor materials to the combined effects of temper embrittlement and hydrogen embrittlement. Accurate estimation of the MPT is important for owners, who are looking for the lowest MPT values to reduce start-up time. Several methods to develop an MPT have been adopted in the past mainly based upon experimental data and/or acquired from material exposed in reactors over time. Most of these methods are based on historical concepts of hydrogen embrittlement in conventional 21/4Cr-1Mo alloys. For 21/4Cr-1Mo-1/4V low alloy steels, hydrogen diffusion and trapping capacity are different and thus, potential for internal cracking is different. In addition, experiments to assess the influence of external hydrogen environment on stable cracking have been reported and provide more insight into stable crack growth potential due to a hydrogen atmosphere. The traditional methods could lead to an overly conservative approach. The result could lead to an overestimation (i.e., a shift to higher temperature envelope) of the MPT curve. In this paper the authors describe a methodology for deriving the MPT for new built 21/4Cr-1Mo-1/4V low alloy steel reactors. It is based on API TR-934 F parts 3 and 4 [1],[2] combined with some practices from WRC Bulletin 562 [3]. A case study will be described and a comparison with results according to historical calculations will be presented.